Hitherto, various imaging elements applicable to such imaging apparatuses have been proposed. Among such imaging elements is an inter-line CCD solid-state imaging element having a vertical overflow drain structure as schematically shown in FIG. 15.
The CCD shown in FIG. 15 comprises a two-dimensional array of photo-diodes 1 arranged in both horizontal and vertical directions and each constituting a photo-electric converting cell or accumulating charge according to light incidence, a plurality of vertical shift registers 3 constituting vertical shift paths for receiving charge accumulated in associated photo-diodes 1 via a transfer gate 2 and progressively vertically shifting the received charge, a horizontal shift register 4 constituting a horizontal shift path for receiving shifted charge from the vertical shift registers 3 and progressively horizontally shifting the received charge, and a signal detector 5 for amplifying the output signal of the horizontal shift register 4 and outputting the amplified signal.
FIG. 16 is a block diagram showing the basic construction of an imaging apparatus with the CCD shown in FIG. 15. The illustrated imaging apparatus comprises a focusing lens 11, a shutter means 12, a CCD 13, a signal processor 14, a shutter driver 16, a signal generator 17, a CPU 18, a lens stop means 20, a recording means 21 and a lens stop driver 22. The focusing lens 11 focuses a light beam of a scene on a light incidence surface of the CCD 13. The lens stop means 20 stops or reduces the area of the light flux of the scene from the lens 11. The shutter means 12 is constituted by, for instance, a mechanical shutter for either passing or blocking the scene light flux. The CCD 1 converts the scene light beam flux having passed through the shutter means 12 to an electric signal. The signal processor 14 performs various processes on the electric signal from the CCD 13 and outputs an image signal thus generated. The recording means 21 has a DRAM for storing the image signal from the signal processor 14 as a still image or a recording medium on which the compresses image signal is recorded as a still image. The shutter driver 16 controls the shutter means 12. The lens stop driver 22 controls the lens stop means 20. The signal generator 17 supplies pulses for controlling the period of charge accumulation in the photo-diodes 1, pulses for driving the vertical shift registers 3 and pulses for driving the horizontal shift registers 3 and pulses for driving the horizontal shift register 4 to the CCD 13 and also supplies pulses for driving the signal processor 14 in synchronism to the CCD 13. The CPU 18 collectively controls circuits including the signal generator 17 and the lens stop driver 22. The signal processor 14 and the signal generator 17 together constitute a digital signal processor (DSP) 19.
FIG. 17 is a timing chart illustrating a conventional imaging operation in the imaging apparatus shown in FIG. 16. Specifically, the Figure shows a vertical sync signal VD, a transfer gate pulse train TG, a sub-pulse train SUB, a vertical shift register shift pulse train VT, a clamp pulse train CLP, opening/closing operation of the shutter means 12, operations of the lens stop means 20 and the lens stop driver means 22 and a CCD signal, i.e., a signal read out from the CCD 13.
The vertical sync signal VD is a pulse train prescribing a predetermined unit period of time for obtaining a signal representing one image (i.e., one frame image). Here, periods prescribed by the individual pulses are labeled V1, V2, . . . .
The transfer gate pulse train TG consists of pulses for determining the timing of the transfer of charge stored in the photo-diodes 1 to the vertical shift registers 3, and is applied to the transfer gate 2 in synchronism to the vertical sync signal VC. The transfer gate pulses TG corresponding to the periods V1, V2, . . . of the vertical sync signal VD are labeled TG0, TG1, . . . .
The sub-pulse train SUB consists of pulses for discharging charge generated in the photo-diodes 1 in the vertical direction of the substrate. The discharge of charge is done while sub-pulses SUB are outputted. That is, the charge is accumulated in the photo-diodes 1 during periods tb1, tb2, . . . , in which the sub-pulses SUB are stopped in the periods V1, V2, . . . of the vertical sync signal VD. Thus, it will be seen that a so-called electron (or element) shutter is realized, in which the effective exposure time is controlled through control of the charge accumulation period. The charge accumulation time is determined as a result of measurement of light of the scene image with a measuring means (not shown), and it is measured by counting sub-pulses SUB.
The vertical shift register shift pulse train VT consists of pulses for causing progressive shift of charge in the vertical shift registers 3 toward the horizontal shift register 4.
The clamp pulse train CLP consists of pulses for clamping portions of the CCD signal corresponding to optical black portion of the CCD. By the clamping, the potential level of the image signal is stabilized to hold a stable black level.
The shutter means 12 is normally open, and is closed (light-shuttered or -blocked) when causing the transfer of charge accumulated in the photo-diodes 1 in response to a recording trigger signal. As the recording trigger signal, in the case of a shutter release button (not shown) providing a two-stage trigger signal, that is, in the case when a first trigger pulse is generated in a preparatory stage of lightly depressing the shutter release button for recording and a second trigger pulse is generated by further depressing the shutter release button for starting the recording of a still image, the second trigger pulse corresponds to the recording trigger signal.
The CCD signal has time sections to1 and to2 corresponding to optical black portions in the vertical direction and an effective time section intervening as a scene image period between these time sections. Normally the optical black signal is at a higher level than the effective period signal level.
The lens stop means 20 is normally in an open diameter state, in the case of such a bright scene that normal exposure will be exceeded with the sole electronic shutter operation in its open diameter state, it is driven to stop the light flux.
As is seen from the timing chart of FIG. 17, in the prior art imaging apparatus upon generation of a recording trigger signal in, for instance, the period V3, vertical shift register shift pulses VT are continuously outputted during a subsequent time section ta for fast sweep-out of unnecessary charge in the vertical shift registers 3, while steadily applying shift pulses without any pause period for the read-out. In the subsequent period V4, the charge is accumulated in the photo-diodes 1 by suspending the application of sub-pulses SUB for a time section tb4 corresponding to the exposure period, which has been determined by a light measurement process executed on the basis of the CCD signal until the recording trigger signal generation. At this time, appropriate exposure may not be ensured with the sole electronic shutter operation. In such a case, in synchronism with the start of the period V4 the lens stop driver 22 is turned on to cause the lens stop means 20 to stop or decrease the diameter of the scene light flux. At any rate, the time section tb4 constitutes an exposure time for one frame image.
In the subsequent period V5, the image obtained by the exposure during the time section tb4 in the period V4 is outputted as signal CCD4, which is outputted as a result of the exposure in response to the recording trigger signal from the signal amplifier 5. Also, in synchronism with the start of the period V5 the lens stop means 20 is driven back to the open state, while the shutter driver 16 is caused to drive the shutter means 12 for closing. In the subsequent period V6, the shutter means 12 is opened. The image obtained by exposure as a result of the closing operation of the shutter means 12 in the period V5 is outputted as signal CCD5 in the subsequent period V6. Since the signal CCD5 is obtained while the shutter means is blocking incident light, the signal levels in the optical black portion time sections and the effective period are substantially equal.
As shown above, in the prior art imaging apparatus, fast sweep-out of charge from the vertical shift registers 3 is executed in the period V3, during which the recording trigger signal is generated, the lens stop means 20 is selectively operated while causing charge accumulation in the photo-diodes 1 during the time section tb4 in the subsequent period V4, the lens stop means 20 is opened while driving the shutter means 12 for closing to cause transfer of the accumulated charge in the subsequent period V5, and the shutter means 12 is opened again in the subsequent period V6.
In the above prior art imaging apparatus, however, a response time tm is required from the start of the closing operation of the shutter means 12 until the perfectly closed state is brought about. In other words, even with the closing operation started at the start of the charge transfer period V5, during the response time tm the light is incident on the CCD 13, resulting in charge generation in the photo-diodes 1. Therefore, particularly in case of a bright scene the charge generated during the response time tm partly enters the vertical shift registers 3 in spite of the charge sweep-out in the vertical direction with sub-pulses SUB. Also, the generated charge remains on the substrate part of the photo-diodes 1, and is shifted by the vertical shift registers 3 after the shutter means 12 has been perfectly closed. Thus, a problem of the superimposition of smear on the intrinsic CCD signal is posed. Here, the lens stop means 20 has a response characteristic similar to that of the shutter means 12.
To solve the problem noted above, the applicant has earlier proposed an imaging apparatus, which has the structure as shown in FIG. 16, and in which the imaging operation is controlled with timings as shown in FIG. 18 (Japanese Patent Application No. 8-344052). In this imaging apparatus, after the recording trigger pulse generation the fast sweep-out of unnecessary charge in the vertical shift registers 3 is executed in a time section tc in synchronism to transfer gate pulse TG3 synchronized to the vertical sync signal VD. The vertical shift register shift pulse VT for the fast sweep-out period tc need not be synchronized to the horizontal blanking period because of the fact that unnecessary charge which is not used as data is swept out.
Transfer gate pulse TG4 prescribes the end instant of the fast sweep-out period tc, and also causes transfer of signal charge having been accumulated during the charge accumulation time section tb4 to the vertical shift registers 3. The timing of generation of the pulse TG4 is set such that it is earlier than the start of the next period V5 by a predetermined time interval tv, which is determined on the basis of the response time tm of the shutter means 12 and an allowance thereof. The shutter driver 16 is thus caused to drive the shutter means 12 for closing in synchronism to the transfer gate pulse TG4.
Furthermore, the vertical shift of the signal charge transferred to the vertical shift registers 3, is suspended for predetermined time tv, and the read-out is started by starting the application of vertical shift register shift pulses VT in synchronism with the end of this vertical shift suspension time tv, i.e., with the start of the next period V5. The timing of the start of the charge accumulation time section tb4 after the generation of the recording trigger signal, is determined to be earlier than the timing of generation of the transfer gate pulse TG4 by the charge accumulation time section tb4. In the case of determining the charge accumulation time section tb4 while causing stopping of the scene light flux, the lens stop driver 22 is turned off such that the lens stopping operation of the lens stop means 20 is caused at the start of the period V4 and turned off in synchronism to the end of the charge accumulation time section tb4, i.e., the transfer gate pulse TG4.
With the imaging apparatus as described, the shutter means 12 can be in the perfectly closed or light-blocked state in the period V5, in which the signal charge accumulated during the charge accumulation time section tb4 in the period V4 is read out. It is thus possible to solve the above problem of smear and obtain high quality image signal.
However, as a result of various experimental researches and investigations conducted by the inventors, it was found that the above imaging apparatus proposed by the applicant has the following problems to be solved. A portable imaging apparatus such as an electronic still camera or a digital camera uses a battery, and consumed (consumption) power reduction is particularly demanded for such imaging apparatus. In the imaging apparatus as described above, the period of driving the lens stop means 20 is the same as the fast sweep-out time section tc for sweeping out unnecessary charge in the CCD 13. For the fast sweep-out, transfer pulses are applied at an iteral frequency (i.e., sweep-out frequency) f, which is usually proportional to peak consumed current I in the CCD 13 at this time as shown in FIG. 19. That is, with increasing sweep-out frequency, the peak consumed current I is increased to increase the consumed power.
Therefore, where the sweep-out frequency f is fixed at a high frequency f1, a very high peak consumed current, which includes the fast sweep-out current for the fast sweep-out and a stop holding current for holding the stopping state of the lens stop means 20, flows through the entire imaging apparatus during the fast sweep-out time section tc as shown in FIG. 18. Particularly, where the lens stop means 20 is normally-open type, a higher peak consumed current flows at the moment of the start of the sweep-out. Such very high peak consumed current increases the power consumption to reduce the battery life and, depending on the battery capacity, reduces the supply voltage, possibly resulting in stoppage of the system operation.
Such a problem is also encountered in the case where an access operation for writing the image data from the CCD 13 is written in the recording means 21 in the fast sweep-out time section tc, and is more readily encountered in the case where the stopping operation of the lens stop means 20 is caused concurrently with such access operation for writing the image data in the recording means 21. Furthermore, a like problem is encountered where the operation of the lens stop means 20 and the fast sweep-out of unnecessary charge are performed at different timings as shown in FIG. 17, the residual battery capacity is less, or the battery capacity is reduced due to a low ambient temperature. Moreover, where signal charge in some lines of the CCD 13 is read out while the charge in the other lines is swept out by fast sweep-out to improve the frame rate during a period from the first trigger pulse till the recording trigger, a like problem is encountered in the case of charging a strobo means or the like concurrently during this time.